(a) Field of the Invention
This invention relates both to apparatus for and methods of counting objects passing a given point, over a period of time. In particular--but not exclusively this invention concerns the counting of relatively thin objects which are advanced through the counting point in an overlapping manner, such as in the case of the counting of printed copy issuing from a printing press.
(b) Description of the Prior Art
Using modern technology, the counting of many kinds of objects passing a given point presents no particular problems, if those objects are spaced apart. For example, the counting can be performed mechanically, optically or magnetically, depending upon the nature of the objects. Even in the case of essentially flat, sheet-like objects which are advanced in an overlapping state, relatively simple counters can produce exact results, provided that the thickness of the objects and their degree of overlap are essentially constant. Unfortunately, for a case where the thickness of the objects is variable or where the extent of the overlap varies, the problem of accurately counting the objects is much increased. Depending upon the method employed, the accuracy may be worsened by the object having variable surface finishes, faults and--for optical counters--colour variations.
A particular problem arises in the case of printed copy leaving a printing press. Such printed copy may be of variable thickness, may have different extents of overlap and may have light and dark printed areas as well as torn or damaged portions. If the printed copy is folded--as in the case for example of newspapers the problems are greatly exacerbated and much effort has been expended on finding satisfactory counters for such printed copy. Whilst mechanical or electro-mechanical counters can sometimes be used to sense the leading folded edge on each printed copy, relatively thin copy cannot be sensed reliably in this way and moreover miscounts can easily occur due to creases, bulges or the like. Experience also shows that closely spaced copies moving at relatively high speeds cannot reliably be counted, even if the mechanical sensors are set and adjusted with great care.
In an attempt to overcome the above problem, there have been proposals for optical photo-electric counters, directing a beam of light obliquely on to the printed copy in the direction advancement of that copy. However, reliable results still are not obtained, either because photo-electric counters can erroneously react to areas of dark print or because the copy is too thin for a reliable output to be obtained.
Improvements in optical counters have been described for example in U.S. Patent Specification No. 4,286,149 (NCR Canada Limited) and in International Specification No. WO 85/05206 (Quantity & Time Management Systems Limited). In both of these Specifications there are described counters employing a pair of beams directed obliquely on to the objects being advanced, from positions upstream and downstream of the counting point. At least one receiver is arranged to detect radiation reflected from the counting point and a determination is made of whether radiation from both sources is being received, or radiation from only one source, on account of a shadow effect at the leading edge of an advancing object. From the output of the detector, a decision is taken on whether the object count should be incremented. In an attempt to improve reliability, the U.S. Specification also describes an arrangement in which two detectors are used, spaced apart by a significant distance along the path of advancement.
Tests have shown that counters as described above still do not give entirely accurate results, especially when used in conjunction with newspapers and similar print copy. Such counters may still be confused by changes in the reflectivity of the copy in the region of the leading edge, caused for example by variations in the density of the printing, and also by variations in the thickness of the copy being counted. Also, erroneous operation frequently occurs when the upper surface of the printed copy is rippled or wavy, such as may happen when inserts, leaflets or the like are placed within the folded printed copy.